Hypothesis: It is well established that rates of major maternal morbidity are highest among women who undergo an attempt at a vaginal birth after a prior c-section (i.e., VBAC, but "fail" that attempt and are delivered by cesarean. Because of this, several investigators have developed scoring systems to predict the labor outcome of women who are attempting a VBAC. Because of both inadequate test characteristics as well as clinical issues with these scoring systems, they have not gained acceptance in the general obstetrical community. A novel test, the fetal-pelvic index (i.e., FPI which combines ultrasound measures of the fetus with x-ray measurements of the maternal pelvis), has been tested in small studies in various groups of women at high-risk for cesarean (including those attempting a VBAC), and appears to have both high sensitivity and specificity. Specific Aims: This proposal has 2 specific aims: 1) To assess the sensitivity and specificity of the fetal-pelvic index (which combines x-ray measures of the pelvis with ultrasound measurements of the fetus) for the success/failure of a trial of labor among those attempting VBAC, and 2) To develop a clinical predictive index to define groups of patients at high risk for a failed trial of labor among women attempting a VBAC. This predictive index could include radiological studies (i.e., the fetal-pelvic index), clinical factors, and historical factors. This clinical predictive index will help clinicians decide whether or not to offer a patient with a prior cesarean section an attempt at a vaginal delivery, based on the risk of a failed attempt at a vaginal delivery to that patient. Validation of the prediction rule will be done internally, using a bootstrapping approach. Methods: We propose a 4-year prospective study to meet the aims of this study. We will enroll pregnant women who have had a prior cesarean and a re planning on a VBAC attempt between 36 and 42 weeks gestation. Patients will be enrolled as outpatients at 2 hospitals-the Hospital of the University of Pennsylvania and Pennsylvania Hospital. Procedures at the time of enrollment will include an in-person interview, review of the prenatal record from the current pregnancy, review of the medical records from all previous deliveries, as well as an ultrasound examination to obtain measurements for the calculation of the FPI. The patient will have the remainder of her care at the discretion of her physicians. Enrolled subjects will have x-ray pelvimetry performed after delivery, but while in the hospital, and these measures will be combined with the ultrasound-based fetal measurements to calculate the FPI. We have chosen to perform the x-ray pelvimetry after delivery (and calculate the FPI post-hoc) because of safety concerns with direct pelvic radiation during pregnancy. Our analysis will consist of descriptive and bivariate analysis, as well as the development of a multivariable clinical predictive index. This index will predict the likelihood of cesarean for any indication in those attempting a VBAC, and could include historical or clinical predictors, as well as the FPI. If the predictive index has high sensitivity and specificity, its implementation could increase the number of women attempting VBAC, reduce the number of failed VBAC attempts, and reduce the total cesarean section rate in this group of women.